In a vibratory compactor of the type to which this invention relates the exciter is enclosed in a housing near the front of the base plate, while the prime mover--usually an internal combustion engine--is carried on the base plate behind the exciter housing and is connected with the exciter as by means of a belt transmission. By reason of its location near the front of the base plate, the exciter has a tendency to propel the machine forward, so that the forces which the operator has to exert upon the handle are mainly for guidance. The housing in which the exciter is enclosed serves as a guard for its rapidly rotating eccentric mass and also comprises a reservoir for a supply of oil by which the exciter bearings are lubricated and cooled. The oil is splashed onto the bearings by the vibration that the exciter generates and the turbulence that its rotation induces in the housing.
Heretofore vibratory compactors have usually been made with either steel base plates or cast base plates. In a compactor with a steel base plate, structure above the base plate that is rigidly secured to it can be welded to it to provide a sturdy, permanent connection. With a cast base plate such structure must be fastened to the base plate by means of bolts or the like, for which aligned holes have to be provided in the connected parts.
A base plate cast from ductile or so-called nodular iron is superior to a steel base plate with respect to wear and abrasion resistance. Heretofore, however, the exciter housing for a compactor having a cast base plate was made as a separate casting that was bolted to the base plate casting. In one version the exciter housing casting was formed as a body having a cavity in its interior that opened outward only through bearing holes in its opposite side walls. Such an exciter housing held only a small quantity of oil. In another version, which held substantially more oil, the base plate casting defined the bottom portion of the exciter housing, and the exciter housing casting and the base plate casting had mating edge surfaces that had to be machined for a close fit to prevent oil leakage out of the housing. In either case the two castings had to be provided with pads for the bolts that fastened them together, and these pads had to be accurately drilled and tapped. Because of the costs of machining, drilling and tapping, and of the fasteners and operations needed for assembly, the cost of a compactor having a cast base plate tended to be higher than that of an equivalent compactor having a steel base plate. In addition, the pads and flanges on the two casting parts, needed for connecting and sealing them, added a substantial amount of weight to the casting structure. As is well known to those familiar with compacting equipment, heavy weight is not in itself a desirable attribute of a vibratory compactor. Compacting effectiveness depends mainly upon the vibratory force that the machine imposes upon the material to be compacted, and in any event detachable weights can be installed on the machine if more weight is desired.
Compactors of the types described above have been manufactured for many years. During all of that time it had been obvious to those concerned with such machines that reduction of the cost and weight of compactors having cast base plates could give them a significant competitive advantage relative to those with steel base plates. Evidently, it was not obvious how this advantage could be attained.
One supposed obstacle to the improvement of compactors with cast base plates related to the assembly of the exciter into its housing. Before the exciter housing casting was fastened to the base plate, the exciter shaft and its eccentric mass were inserted into the housing casting through coaxial bearing bores in its side walls and the bearings for the exciter shaft were press fitted into those bores. It seemed obvious that forming the exciter housing in one piece with the base plate would have complicated factory assembly and would have made field replacement of the press fitted bearings difficult or impossible, owing to the need for manipulating the whole large, heavy and awkwardly shaped base plate casting and correctly positioning and supporting it in an arbor press for installation of the exciter shaft bearings.